This invention relates to a method and apparatus for detecting blockage in a vapor flow line such a vapor return line in a liquid dispensing and vapor recovery system. Such systems are now commonly used in gasoline service stations in which liquid gasoline is dispensed from an underground storage tank to a container while vapors from the container are drawn back to the storage tank. With the recent increased emphasis on preventing pollution, attention has been directed to minimizing the escape of gasoline vapors to the atmosphere from both permanent storage tanks and vehicles into which gasoline is dispensed.
Satisfactory recovery of gasoline vapors from underground storage tanks can be achieved by providing a separate vapor return line connecting the storage tank to the tank truck which periodically fills the storage tank. In this manner liquid gasoline introduced into the storage tank from the tank truck displaces vapors in the storage tank and forces them through the recovery line to the tank truck to be ultimately disposed of either by burning or through compression-refrigeration systems. Because of the simple connections, vapor line blockage is usually not a problem.
Satisfactory systems for recovering vapors from the gasoline tanks of vehicles, however, are more difficult to design. One such system is the balanced displacement system in which gasoline entering a vehicle tank forces the vapors through a separate line to the storage tank. Other such systems employ vacuum pumps or blowers to remove the vapors from vehicle tanks and to return the vapors to the storage tank. These and similar systems all have in common a fluid line for communicating vapor from the vehicle tank back to the underground storage tank. The effectiveness of these vapor recovery systems can be defeated if the vapor return line becomes partially or completely blocked. For example, fuel may condense creating a liquid blockage in a vapor return hose line between the dispensing nozzle and the dispenser station. Blockage may also occur in a vapor return line between the dispenser station and the underground storage tank due, for example, to ground shift, liquid accumulation at a low spot or solid blockage. Blockage may also occur due to overfilling of the underground storage tank from a tank truck.
Because any of these types of blockages can diminish or defeat the effectiveness of the vapor recovery system, it is desirable to automatically monitor vapor return lines in order to determine whether a blockage has occurred. In fact, various governmental regulatory organizations such as the State of California, Air Resources Boards, have issued requirements for such systems. See, for example a document titled "Certification Procedures for Gasoline Vapor Recovery Systems at Service Stations" issued by the California Air Resources Board on Mar. 30, 1976.